第一原理理論計算研究鐵/氧化鎂/鐵加覆蓋層的穿隧結之垂直磁異向性與穿隧磁阻

First-Principles Studies of Perpendicular Magnetic Anisotropy and Tunneling Magnetoresistance of Capped Fe/MgO/Fe Tunnel Junctions

林書緯(Shu-Wei Lin)

指導教授 ： 郭光宇

國立臺灣大學/理學院/物理研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.00030

摘要

近年來，人們發現鐵/鉻/鐵三層材料組成的結構物質具有巨磁阻的現象。巨磁阻為磁阻效應的一種，可以在磁性材料和非磁性材料相間的薄膜層結構中觀察到。這種結構物的電阻值與鐵磁性材料薄膜層的相對磁化方向有關，兩層磁性材料磁化方向相反情況下的電阻值，明顯大於磁化方向相同時的電阻值，使得電阻在微弱的外加磁場下具有很大的變化量。巨磁阻效應已被成功地運用在硬碟生產上，具有重要的商業應用價值，而造成巨磁阻的現象主要是由於鐵磁材料其自旋能帶在費米能級附近分裂的特性，造成自旋態密度在費米能級的差異。由於巨磁阻的發現，用來操縱電子自旋自由度的自旋電子學成為物理上重要的一項領域。
我們利用第一原理計算探討鐵/氧化鎂/鐵穿隧結具有高垂直磁異向能以及高穿隧磁阻的物理原因。在我們的研究中，這是由於在鐵/氧化鎂界面上氧化鎂中的氧與鐵鍵結，導致界面鐵具有一個高自旋極化率的表面態，另外由於在界面上對稱性的破壞，導致界面鐵的能帶由於自旋軌道耦合效應產生分裂的現象，進而發生系統能量對磁矩方向的相關性。第二部分，我們使用第一原理理論計算鐵/氧化鎂/鐵穿隧結在外加覆蓋層(cap)後，對其所造成的物理性質變化。我們分別計算了氧化鎂、釕、鉭，這三種材料並且探討其穿隧磁阻與垂直磁異向性的變化，我們發現這三種材料在與鐵的界面上分別呈現非磁性、鐵磁以及反鐵磁的耦合效應，其中反鐵磁性的鉭對於鐵/氧化鎂/鐵穿隧結的穿隧磁阻具有最好的增強效果，而非磁性絕緣體的氧化鎂對於垂直磁異向能則具有最佳的表現。

關鍵字

自旋電子學 ； 穿隧磁阻 ； 磁穿隧結 ； 自旋軌道耦合效應 ； 磁異向性 ； 第一原理計算

並列摘要

Recent studies have reported giant magnetoresistance (GMR) in Fe/Cr/Fe trilayer systems. This phenomenon usually occurs in systems consisting of two magnetic electrodes separated by one nonmagnetic metallic layer. Due to the dependence of the electric resistance on the relative magnetization directions in the two magnetic electrodes, the antiparallel magnetization can produce an electric resistance that is stronger than the parallel magnetization. Thus, applying a weak magnetic field can cause a large variation of electric resistance. This dependence of electric resistance on magnetization directions originates from the electronic energy band that splits near the Fermi level in magnetic materials, which results in the different spin density of states at Fermi level. Therefore, the discovery of GMR brought about the spintronics, which exploits the spin-degree of freedom, to be an important branch of physics.
We have performed the first principle calculations to study the physical properties of the Fe/MgO/Fe trilayer system, which has been shown to have the large perpendicular magnetic anisotropy (PMA) and tunneling magnetoresistance (TMR). In the present work, these properties are due to the hybridization between the iron and the oxygen at the Fe/MgO interface, which leads to a surface state with a large spin-polarization. Furthermore, due to the broken symmetry at that interface, if the spin-orbit coupling is taken into account, the energy band of the interfacial Fe would split, leading to an energy dependence of the magnetization direction. In the second part of this thesis, the first principle calculation is performed to study the influences on PMA and TMR when the Fe/MgO/Fe is capped with some materials. Here the MgO, Ta and Ru capping layers are considered, and they reveals nonmagnetic, antiferromagnetic and ferromagnetic coupling to the Fe layer, respectively. In the Fe/MgO/Fe trilayer systems, TMR is usually largest in the Ta-cap systems whereas PMA is usually largest in the MgO-cap systems.

並列關鍵字

Spintronics ； Tunneling magnetoresistance ； Magnetic tunnel junction ； Spin-orbit coupling ； Magnetic anisotropy ； First principles calculation

參考文獻

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